Exponential Growth
Exponential growth is when something doubles over and over, spreading faster each time.
Think of it this way
You place one grain of rice in the first square of a tray. In the next square, you place two grains. Each square gets double what the square before it had. By the tenth square, you have over 500 grains in that one square alone. By the twentieth, the tray cannot hold the pile.
Explaining exponential growth by grade level
Think about one sick person who passes a cold to two friends. Now those two each pass it to two more. That makes four sick people, then eight, then sixteen. It starts slow, but the number grows very fast.